Much previous research has supported the idea that the uptake of oxygen by human erythrocytes in the lung is partly limited by the diffusion resistance of the red cell membrane to oxygen and that intracellular diffusion of oxygen and the rate of reaction of oxygen with hemoglobin are not rate-limiting. Other experiments, however, lead to the conclusion that the plasma membrane of the red cell is so permeable to oxygen that it should not limit the rate of oxygen uptake at all. An effective unstirred layer of plasma surrounding the red cell is another factor that might partly limit the rate of oxygen uptake by the cell. While the role of the unstirred layer has been given relatively little attention, there is experimental work that supports the idea that even in the extremely turbulent flow that is present in the rapid reaction apparatus there is an effective unstirred layer around the red cell of such dimensions that it might partly limit the rate of oxygen uptake. The aim of this project is to precisely define the role of the red cell plasma membrane and the influence of the unstirred layer on the kinetics of oxygen uptake by the human erythrocyte. By coming to grips with the influences of the red cell membrane and the unstirred layer on the kinetics of oxygen uptake, we will contribute to enhanced understanding of the respiratory function of the human erythrocyte.